While I'm sure that all these "ha ha only serious" replies you're getting are amusing (I had a good laugh over the bit about severance pay), this unfortunately doesn't solve your problem. Then again, I'm not sure your problem has a solution.
I absolutely agree that it's time to update your resume. You might think about checking "no" in the "May we contact your employer?" box when applying at a new company in this case. In fact, you should start [b]aggressively[/b] seeking new employment opportunities at this time. You're moving into a no-win situation with this employer.
The reason you probably won't get any liability release forms for something like this is that nobody that knows anything useful about system security ever has the problem of needing such a liability release form. Such people have a tendency to just walk away from a job like that. Furthermore, I'm not sure that a liability release like the one for which you're asking would stand up in court under most circumstances. The only thing it's likely to do for you, in practical terms, is piss off your employer. What it's likely to do [b]to[/b] you, on the other hand, is provide proof positive in court that both you and your employer knew that what you were doing might lead to actionable problems, thus actually doing the opposite of what you'd like to happen with a liability release.
Get out. Get out now. Run, don't walk, to a new employer. You have my sympathies.